This invention relates to a multi-level circuit of the kind having on a substrate at least one conductive track crossing over at least one other conductive track with insulative material therebetween at the cross-over region. The invention relates especially to a method of fabricating such a circuit and also to a display device incorporating the circuit.
A circuit of the aforementioned kind may be used in a variety of different products. For example, such a circuit is used in an active matrix-addressed electro-optic display device of the type in which a matrix array of columns and rows of picture elements is defined by opposing electrodes carried on respective substrates between which, for example, liquid crystal material is present. One substrate carries an electrode common to all picture elements while the other substrate carries an array of individual picture element electrodes each associated with a switching element, such as a thin film transistor (TFT). The switching elements are driven via sets of column and row conductors which cross one another. Scanning (gating) signals are applied to the row conductors sequentially so as to turn on the switching elements of each row in turn while data (e.g. video) signals are applied to the column conductors for transfer to the appropriate picture element electrodes via the switching elements.
The substrate carrying the sets of row and column conductors constitutes a multi-level circuit of the aforementioned kind. The circuit usually is formed by defining two sets of parallel conductive tracks on the substrate crossing at right angles to one another and separated at least at the regions of the cross-overs by insulative material. Apart from their cross-over regions, the conductors of each set lie generally in respective planes overlying one another and substantially parallel with the substrate surface.
This type of display device can be used to provide TV displays, and it will be appreciated that for this purpose a large number of picture elements, for example, 150,000, is required; and that a correspondingly large number of cross-overs between the row and column conductors is involved.
A problem commonly experienced in production of these circuits for matrix display devices is that a short may occur between a row conductor and a column conductor at their cross-over, mainly due to defects in the insulative separator. A cross-over short is a serious problem and can prove catastrophic as it may result in part or all of a column or row of picture elements being faulty. These cross-over shorts have, therefore, a significant effect on yields.
In an attempt to overcome this problem, it has been proposed that a technique such as laser scribing be used to cut through a conductor on either side of the cross-over fault, so as to isolate the cross-over electrically, and to connect both ends of the conductors to the drive circuit so that the remaining portions of the row or column concerned can still function. However, this is an expensive and time consuming process and it is necessary to identify the, or each, cross-over fault before remedial action can be taken.